1. Field of the Invention
The present invention relates generally to lithographic projection apparatus and more particularly to lithographic projection apparatus including imaging mirrors.
2. Description of the Related Art
Lithographic projection apparatus generally include a radiation system for supplying a projection beam of radiation, a support structure for supporting patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern, a substrate table for holding a substrate and a projection system for projecting the patterned beam onto a target portion of the substrate.
This application claims priority from EP 00309871.2 filed Nov. 7, 2000 which is incorporated by reference herein in its entirety.
The term xe2x80x9cpatterning structurexe2x80x9d as here employed should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (IC.s). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
No material suitable for manufacturing refractive lenses usable with EUV radiation is known. Accordingly, a projection system for a lithographic apparatus making use of EUV radiation for the projection beam must be based on reflective optics, generally with multi-layer coated mirrors. Projection systems for EUV radiation have been proposed, for example in: xe2x80x9cDesign approach and comparison of projection cameras for EUV lithographyxe2x80x9d, Lerner et al Opt. Eng. 39(3) 792-802) March 2000; WO99/57596 (Braat); WO99/57606 (Braat), U.S. Pat. No. 5,686,728 (Shafer) and U.S. Pat. No. 5,815,310 (Williamson). These systems have various shortcomings, such as being far from telecentric or having very little working space, and a need exists for alternative systems. In a classification system described below, the Braat six-mirror systems fall into class 41 (+) and the Williamson six-mirror design falls into class 45(xe2x88x92). The four-mirror systems described by Lerner et al fall into classes 9(+) and 10(xe2x88x92). The six- and eight-mirror systems described by Shafer fall into classes 41(+) and 165(+).
One aspect of the present invention provides alternative and improved projection systems for EUV radiation and a methodology for designing such systems.
According to a first aspect of the present invention there is provided a lithographic projection apparatus comprising:
a radiation system for providing a projection beam of radiation;
a support structure for supporting patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern;
a substrate table for holding a substrate;
a projection system for projecting the patterned beam onto a target portion of the substrate,
characterized in that:
said projection system has precisely four imaging mirrors in the optical path of the projection beam and has an incidence angle classification, C, of 2(xe2x88x92), 6(xe2x88x92), or 9(xe2x88x92), where:   C  =            ∑              i        =        1            4        ⁢                            a          i                ·                  2                      (                          4              -              i                        )                              ⁢              (                  M                      "LeftBracketingBar"            M            "RightBracketingBar"                          )            
ai=1 if the angle of incidence of the chief ray at mirror i is negative,
ai=0 if the angle of incidence of the chief ray at mirror i is positive,
M is the magnification of the projection system, and
the index i numbers the mirrors from object to image.
According to a second aspect of the present invention there is provided a lithographic projection apparatus comprising:
a radiation system for providing a projection beam of radiation;
a support structure for supporting patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern;
a substrate table for holding a substrate;
a projection system for projecting the patterned beam onto a target portion of the substrate,
characterized in that:
said projection system has precisely six imaging mirrors in the optical path of the projection beam and has an incidence angle classification, C, of 5(+), 6(xe2x88x92), 9(+), 13(+), 18(xe2x88x92), 21 (+), 22(xe2x88x92), 25(+), 29(+), 34(xe2x88x92), 37(+), 38(xe2x88x92), 42(xe2x88x92), or 54(xe2x88x92), where:   C  =            ∑              i        =        1            6        ⁢                            a          i                ·                  2                      (                          6              -              i                        )                              ⁢              (                  M                      "LeftBracketingBar"            M            "RightBracketingBar"                          )            
ai=1 if the angle of incidence of the chief ray at mirror i is negative,
ai=0 if the angle of incidence of the chief ray at mirror i is positive,
M is the magnification of the projection system, and
the index i numbers the mirrors from object to image.
According to a third aspect of the present invention there is provided a lithographic projection apparatus comprising:
a radiation system for providing a projection beam of radiation;
a support structure for supporting patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern;
a substrate table for holding a substrate;
a projection system for projecting the patterned beam onto a target portion of the substrate,
characterized in that:
said projection system has precisely eight imaging mirrors in the optical path of the projection beam and has an incidence angle classification, C, of 2(+), 5(+), 9(+), 12(+), 13(+), 18(+), 18(xe2x88x92), 19(+), 20(+), 21(+), 22(+), 23(+), 25(+), 26(+), 26(+), 34(xe2x88x92), 36(+), 37(+), 38(xe2x88x92), 45(+), 46(+), 49(+), 52(+), 53(+), 54(+), 54(xe2x88x92), 55(xe2x88x92), 58(xe2x88x92), 68(+), 69(+), 73(+), 74(+), 77(+), 82(+), 82(xe2x88x92), 85(+), 88(+), 89(+), 90(xe2x88x92), 92(+), 93(+), 97(+), 100(xe2x88x92), 101(+), 102(xe2x88x92), 104(+), 105(+), 106(+), 106(xe2x88x92), 107(+), 108(+), 109(+), 109(xe2x88x92), 110(+), 110(xe2x88x92), 111(+), 113(+), 116(+), 117(+), 118(+), 118(xe2x88x92), 120(+), 121(+), 122(xe2x88x92), 123(xe2x88x92), 132(+), 133(+), 134(xe2x88x92), 137(+), 138(+), 141(+), 145(+), 145(xe2x88x92), 146(+), 146(xe2x88x92), 147(+), 148(+), 148(xe2x88x92), 149(+), 150(+), 152(xe2x88x92), 153(+), 154(+), 154(xe2x88x92), 155(+), 155(xe2x88x92), 156(+), 157(+), 159(+), 161(+), 162(xe2x88x92), 163(xe2x88x92), 164(+), 165(+), 166(+), 166(xe2x88x92), 167(+), 168(+), 169(+), 170(+), 170(xe2x88x92), 171(+), 172(+), 173(+), 174(+), 175(+), 176(+), 177(+), 178(xe2x88x92), 179(+), 180(+), 180(xe2x88x92), 181(+), 182(+), 182(xe2x88x92), 183(+), 183(xe2x88x92), 184(+), 185(+), 185(xe2x88x92), 186(xe2x88x92), 187(+), 187(xe2x88x92), 188(xe2x88x92), 189(+), 196(+), 197(+), 201(+), 203(+), 205(+), 209(+), 214(xe2x88x92), 216(+), 217(+), 218(+), 218(+), 218(xe2x88x92), 225(+), 228(+), 229(+), 30(+), 232(+), 233(+), 235(+), 236(+), 237(+), 238(xe2x88x92), 243(+), 246(+), 247(+), 248(+), 250(xe2x88x92), where:   C  =            ∑              i        =        1            8        ⁢                            a          i                ·                  2                      (                          8              -              i                        )                              ⁢              (                  M                      "LeftBracketingBar"            M            "RightBracketingBar"                          )            
ai=1 if the angle of incidence of the chief ray at mirror i is negative,
ai=0 if the angle of incidence of the chief ray at mirror i is positive,
M is the magnification of the projection system, and
the index i numbers the mirrors from object to image.
An embodiment of the present invention may comprise a four-mirror projection system in class 6(xe2x88x92) with a numerical aperture of 0.15, a ring field between xe2x88x9222.8 mm and xe2x88x9223.8 mm on the image side, and a transverse magnification of xe2x88x920.2 at a wavelength of 13 nm. Such a system can have a minimum Strehl ratio of 0.972, a maximal wavefront error of 0.0266 waves and a maximal distortion of 12 nm.
The present invention, in a fourth aspect also provides a device manufacturing method using a lithography apparatus comprising:
an illumination system constructed and arranged to supply a projection beam of radiation;
a first object table constructed to hold a mask;
a second object table constructed to hold a substrate; and
a projection system constructed and arranged to image an irradiated portion of the mask onto target areas of the substrate; the method comprising the steps of:
providing a mask containing a pattern to said first object table;
providing a substrate which is at least partially covered by a layer of radiation-sensitive material to said second object table;
irradiating portions of the mask and imaging said irradiated portions of said mask onto said target areas of said substrate; characterized in that:
in the step of imaging, a projection system as defined in any one of the first, second and third aspects described above is used.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skill ed artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm), as well as particle beams, such as ion beams or electron beams.